Kernel Threads:-

A kernel thread is a way to implement background tasks inside the kernel. The task can be busy handling asynchronous events or sleep-waiting for an event to occur. Kernel threads are similar to user processes, except that they live in kernel space and have access to kernel functions and data structures. Like user processes, kernel threads have the illusion of monopolizing the processor because of preemptive scheduling. Many device drivers utilize the services of kernel threads to implement assistant or helper tasks. For example, the khubd kernel thread, which is part of the Linux USB driver core monitors USB hubs and configures USB devices when they are hot-plugged into the system.

Creating a Kernel Thread

Let's learn about kernel threads with the help of an example. While developing the example thread, you will also learn about kernel concepts such as process states, wait queues, and user mode helpers. When you are comfortable with kernel threads, you can use them as a test vehicle for carrying out various experiments within the kernel.

Assume that you would like the kernel to asynchronously invoke a user mode program to send you an email or pager alert, whenever it senses that the health of certain key kernel data structures is deteriorating. (For instance, free space in network receive buffers has dipped below a low watermark.)

This is a candidate for being implemented as a kernel thread for the following reasons:

1. it’s a background task because it has to wait for asynchronous events.

2. It needs access to kernel data structures because the actual detection of events is done by other parts of the kernel.

3. It has to invoke a user mode helper program, which is a time-consuming operation.

Our example kernel thread relinquishes the processor until it gets woken up by parts of the kernel responsible for monitoring the data structures of interest. When awake, it invokes a user mode helper program and passes appropriate identity codes in its environment.